Known worm gearings of this type serve to transmit relatively small torques. In the gearing arrangement according to German Patent No. 253,658, which for example is used for driving a contact roller for advertisement illumination purposes, the worm grips around the worm gear over 180.degree. of the periphery thereof. The worm gear consists of a soft material, into which the worm windings are to cut grooves. In a further known gearing (U.S. Pat. No. 3,220,278), which for example is used for driving a tape recorder, the worm gear has a plurality of teeth right from the start. The worm surrounds the worm gear over an angle of 270.degree.. A special advantage of such worm gearings is that a larger number of threads can simultaneously engage the worm gear. However, a gearing of the above-mentioned type is also known (U.S. Pat. No. 2,635,479 ), in which the worm has a straight axis. The worm can for the purpose of an uncoupling be released by means of a movement mechanism from its engagement with the worm gear. The elasticity of the worm is to prevent impacts during recoupling.
It has been proven that the known gearings of the abovementioned type are only suited for transmitting larger torques, if a longer lifetime of the gearings is not important. The lifetime of the gearings is limited because the worm, depending on the direction of rotation, is either pulled together or widened. Both the diameter and also the thread change hereby, through which a clamping occurs between the worm thread and the teeth of the worm gear. This leads relatively quickly to a breakage of the worm so that such gearings do not reach the service life which is necessary, for example for a car window gearing or other adjusting gearings, for example for roller shutter operations.
The basic purpose of the invention is to construct in a gearing of the above-mentioned type a worm so that the windings can no longer shift relative to one another.
This purpose is attained according to the invention by the strandlike member having two rows of projections and recesses, whereby in wound-up condition of the strandlike member the projections of the one row engage the recesses of the other row to prevent a relative reciprocal shifting between the adjacent windings.
In a so constructed worm gearing, a reciprocal shifting of the windings is neither possible in the sense of a pulling together of the worm nor in the sense of a widening of the worm, since the positive engagement of the projections in the recesses prevent reciprocal shiftings between the windings. Thus the worm maintains at all times its diameter and therefore fits with the provided clearance into the teeth of the worm gear, so that damaging jammings are avoided. Through this a much greater lifetime for the worm can be achieved.
The projections and recesses may have many different shapes. For example, tooth profiles are advantageous so that so to speak two racks are provided on the strandlike member.
The projections and recesses of the one row can be offset with respect to the projections and recesses of the other row, however, they must not have such an offset. The offset depends on the desired diameter of the worm and, therefore, differs from case-to-case.
The projections and recesses can be constructed wedge-shaped in such a manner that the spacing between the side edges of the projections, seen in peripheral direction of the worm and which lie opposite to one another, decreases from the outside edge thereof to the inside edge. As a result, radial planes on the side surfaces of the projections and recesses which go through the axis of the worm extend fan-shaped with respect to one another, so that an abutment of the projections in the recesses can only be accomplished over the entire radial thickness of said projections and recesses by the utilization of a wedge shape. However, one can also do without this, in particular when the radial thickness of the projections and recesses is small in relationship to the diameter of the worm.
The strandlike member may have an approximately T-shaped cross section, whereby the T-shaped strand forms the teeth engaging the worm gear teeth and the T-crossbeam forms a foot flange lengthwise along the two side edges at which the projection/recess rows are arranged. Such a strandlike member can be wound up particularly well to a worm, because the foot flange forms a good abutment surface for resting on a winding mandrel.
In a different embodiment of the invention, the strandlike member has an approximately cross-shaped cross section, whereby one crossbar forms the thread engaging the worm gear teeth and two crossbars which lie opposite one another form flanges, lengthwise the edges of which the projection/recess rows are arranged. Such a strandlike member is particularly advantageous with respect to the rigidity thereof, because fixation against shifting lies close to the stressed threads. The worm may be constructed both with one thread or also with several threads. In the latter case, several strandlike members are wound-up parallel with one another. By using worms which have a plurality of threads thereon enables the transmission of large forces.
The axis of the worm may be straight. In this case, the advantage is utilized that the worm can be manufactured inexpensively. In a preferred embodiment of the invention, however, the axis of the worm is curved over at least a portion of its length so that several threads simultaneously engage the worm gear. Here the advantage mentioned in the beginning is utilized, that a flexible worm can be adjusted to the worm gear. It is then possible to transmit with a relatively thin worm relatively large torques, so that the gearing can be constructed very flat.
The strandlike member may consist of any suitable material. Suitable is for example, metal, in particular steel, if we deal with the transmission of greater forces. In transmitting smaller forces, nonmetallic materials can also be used, thus, for example, plastic.